Trinucleotide repeat (TNR) expansion mutation is genetic instability that is the known cause for many inherited neurodegenerative diseases. Variety of experimental systems have been developed to shed light on features of the expansion mechanism. In rapidly dividing bacteria and yeast long TNR have been demonstrated to be unstable due to their ability to form alternative secondary structures such as hairpins. However, unlike in mammals, long TNR in these simple organisms undergo contraction rather than expansion unless specific selection conditions are introduced. Mouse in vivo models for expansion diseases, on the other hand, have allowed to uncover molecular features of mutation associated with development and aging. Expansion in mammals has been found to occur during germ cell development and with age in affected by the disease areas of the brain. Both events require the presence of mismatch repair enzyme Msh2. We and others have demonstrated that Msh2- dependent expansion in germ and somatic cells arises from repair at a DNA break rather than during mitosis. In contrast, contraction of the long repeats in mice can occur during replicatiive phase in a very short time window in early embryogenesis. It is not known whether establishment of checkpoint proteins and signaling to DNA repair machinery is important in determining the fate of TNR in early organism development. It is a goal of this proposal to determine: i) what is the role of checkpoint proteins during proliferative phase of mammalian development in driving instability of TNR and ii) whether manipulations with checkpoint proteins in the early development can shift repeat number towards contractions. Public Health Relevance: Trinucleotide repeat expansion mutation is the cause of a number of inherited neurodegenerative diseases. This is dynamic mutation which worsens in succeeding generations. Expansion has been shown to occur in germ cells of the transmitting parent and in the affected by the disease areas of the brain of the offspring. Two cellular processes have been demonstrated in different model organisms to contribute to repeat expansion, that is DNA replication and DNA repair. These processes are likely differentially used in dividing and non-dividing cells. We have shown previously that repeat expansion occurs in non-dividing cells as they try to repair DNA damage while repeat contraction can occur in dividing cells and involves DNA replication. The latter is limited to a very short time window in early embryogenesis. It is not known how the machinery which regulates cell division,-cell cycle control,-affects stability of the triplet repeats in early development. In this proposal we will determine the role of cell cycle control proteins in triplet repeat instability by establishing mouse embryonic stem cell lines and studying changes in repeat length during cell division (Aim 1). Using this model we will test whether manipulations with cell cycle control machinery (Aim 2) can shift repeat number towards contractions that is advantageous for the survival of the cells which carry long triplet repeats. The proposed experiments will advance our understanding of the mechanism of triplet repeat instability and point towards avenues to explore molecular targets important for preventing expansion.